Multiple classes:
-Jar Glitch: Affects Mad Milk + Jarate, easy to reproduce. 1) Throw jar, 2) touch resupply, 3) throw Jar again. Your jar will (contrary to the HUD) refresh from the time you threw the first jar, not the second one. This essentially lets you throw jar items faster than normal.
Soldier:
-Beggar's Bazooka has a reload glitch that prevents overcharge damage, to replicate just hold left click and release really fast over and over.
Demoman:
-Not really a bug but I'm sure people would appreciate it if Valve tuned pipe damage so that the amount of direct pipes to kill a Soldier wasn't somewhat random between 2 or 3.
-Charge scripts still exist thanks to abusing Novint Falcon stuff.
Heavy:
-The GRU death mark occasionally shows above the Heavy head even after he switches away from the GRU.
-Not sure if bug but if the Holiday Punch was made to not work on ubercharge's it would be unbanned in UGC.
Medic:
-If you quickswitch your Syringe Gun or Melee weapons to another weapon you get to switch spawns and not lose your Ubercharge %. AFAIK this is banned in every league but it's pretty overpowered and kind of contrary to every other item that has a %.
Engineer:
-If a Spy applies a sapper to a building during the building's upgrade animation, it can not be unsapped by the Engineer or Pyro.
Sniper:
-Bazaar Bargin has a bug where on a no-crit server after you get 1 head it will continuously give you more and more heads for just shooting nothing.
Spy:
-A disguised Spy using the in-game voice command menu will occasionally display as his real name + real team color to the enemy team in chat if he is nearby any of them, instead of disguised name + disguised team. (since you can't see enemy team voice commands, this is basically an instant give away)
-If you disguise as a player as they change classes, you will occasionally get a disguise with their new classes HP (e.g.: A Scout Disguise with 300 HP)
-If you pick up a med kit while disguised the healing pickup will be your real team color if your disguise is full health instead of disguised team color.
-Applying the same disguise or touching a resupply while disguised changes your disguised weapon to melee.